This invention relates generally to an electrical circuit element that conducts current with a low resistance during normal operation but, during a significant current increase, a significant ambient temperature increase, or some combination of current and ambient temperature increases, will transition to a high resistance state and limit the current flowing through the circuit. Once the current and/or ambient temperature overload condition passes, the element may return to its normal low resistance mode of operation so that a resettable fuse element is formed. In more detail, a polymer-based current limiting device and a method of manufacture is described.
An electrical circuit element that can be tripped and then be reset is well known. For example, there are many different current limiting devices wherein the device permits current to pass during normal circuit operation. The device is made of materials which exhibit a property that, during periods of increasing current, the materials melt due to the heat generated by the increased current and the resistance of the device increases so that the current is effectively limited. Once the over-current condition subsides, the materials in the device solidify again and the device returns to its original mode of operation.
There are many different conventional current limiting devices. For example, U.S. Pat. No. 2,258,958 to Pearson, assigned to Bell Labs, describes a conductive device wherein the conductance varies as a function of applied voltage or current, which may be used in the regulation and control of electric current. The device has conducting particles suspended in an insulating matrix where the coefficients of thermal expansion are very different. This appears to be an early patent on positive temperature coefficient (PTC) polymeric current limiters (PCL). Another patent recites a regulator device for electric current, which is PTC, resettable, trips rapidly, and trips with overcurrent and/or overtemperature. This patent recites carbon particles dispersed in a mixture of polyethylene (PE) and polytetrafluoroethylene (PTFE). Another patent recites using different materials to produce a heater cable wherein a carbon black/polymer composite heater cable obeys an arbitrary relation.
Raychem has several patents in the area of current limiting devices. For example, one patent recites low resistivity PTC compositions wherein a particular range of carbon black properties, composite resistivities, and temperatures are specified for conductive polymer composites, especially for circuit protection devices. The patent lists fluoropolymers as one possible polymer component. Another Raychem patent recites a method for annealing PTC compositions wherein conductive polymer composites containing two polymers have improved electrical properties are produced when the composite is annealed at a temperature between the melting points of the two polymers. Another Raychem patent recites a self-regulating heater cable made by winding a conductive polymer composite strand around two parallel wires, candy-cane fashion. Other Raychem patents recite different methods for manufacturing current limiting devices. Other Raychem patents cite high temperature current limiting devices wherein a mixture of PTFE and a fluoropolymer is used. Other companies have similar patents, which recite and describe other configurations of current limiting devices. Another Raychem patent to Lunk et al. describes devices made from perfluoroalkoxy (PFA) polymer, shows resistance versus temperature data, and discusses the potential use as current limiting devices in higher temperature environments.
In addition, Therm-O-Disc has received several patents for current limiting devices with a similar higher temperature range, based on nylon which has a melting point of up to 190xc2x0 C. These patents actually teach away from using fluoropolymers to make polymer current limiting devices. Therm-O-Disc has also received a patent that, in part, describes the use of a high temperature solder for attaching electrical wires to a current limiting device that improves the properties of the device. The patent names various solders, but the best material (e.g., Sn95Ag5) has a high melting point of only 245xc2x0 C.
Despite the large amount of conventional current limiting devices described in the literature and in various patents, the conventional current limiting devices do not achieve the advantages of the PCL device in accordance with the invention. In particular, none of the conventional current limiting devices is capable of operating at the wider temperature range and none of the conventional devices have the low thermal derating. Thus, it is desirable to provide a polymer current limiting device and it is to this end that the present invention is directed.
The polymer current limiting device (PCL) in accordance with the invention overcomes the problems and limitations with existing commercially available current limiting devices. In addition, the PCL in accordance with the invention has various advantages over the existing current limiting devices. The operating temperature range of the PCL device in accordance with the invention is significantly wider. The PCL devices have been experimentally shown to work as current limiters over the temperature range of xe2x88x9260xc2x0 C. to 280xc2x0 C. Most commercially available current limiting devices have a rated temperature range of xe2x88x9240xc2x0 C. to 85xc2x0 C. although some new devices have a rated temperature range of xe2x88x9240xc2x0 C. to 125xc2x0 C. A higher maximum operating temperature is important because it allows the PCL devices in accordance with the invention to be used in environments where the ambient temperature is very hot, perhaps too hot for existing commercially available PCL devices. For example, the PCL devices in accordance with the invention may be used near the engine or under the hood of a car, in electronics that may be near fire, in the door of a car parked in the sun in Arizona, outdoor use in hot climates, in electronics used in well drilling and geothermal applications, electronics used near steam and hot fluids, etc.
In addition, the preferred polymer used in the PCL devices in accordance with the invention (perfluoroalkoxy (PFA) polymer) is nonflammable and self-extinguishing in a fire. The polymer used in existing current limiting devices is flammable and will burn on its own after being ignited by a flame, a spark, or by self-heating during normal use in a circuit which experiences a large overcurrent. This makes the PCL devices in accordance with the invention much less of a fire hazard than existing current limiting devices. This also allows the PCL devices to be used in environments where there are sparks, such as a conventional circuit breaker, or flames, such as a water heater.
In addition, the PCL devices in accordance with the invention have a significantly better xe2x80x9cthermal deratingxe2x80x9d than other commercially available current limiting devices wherein the thermal derating is the rate at which the device trip current decreases as the ambient temperature increases. As a quantitative example, most Raychem current limiting devices (those with an operating temperature range of xe2x88x9240xc2x0 C. to 85xc2x0 C.) have a thermal derating of xe2x88x921%/xc2x0C., meaning that the trip current decreases by an average of 1% for every 1xc2x0 C. increase in the ambient temperature. The PCL devices in accordance with the invention have an experimentally measured thermal derating of xe2x88x920.4%/xc2x0C. over the same operating temperature range meaning that the trip current decreases by an average of 0.4% for every 1xc2x0 C. increase in the ambient temperature. This is commercially useful because it makes the PCL devices more tolerant to changes in ambient temperature and fluctuations in the thermal environment.
In accordance with the invention, the PCL devices may preferably be made with pure lead (Pb) solder joints between the foil electrodes and the lead wires. Pure lead (Pb) solder joints have a nominal melting point of 327xc2x0 C. Using the pure lead (Pb) joints, the PCL devices still function for brief temperature excursions up to 350xc2x0 C. Most commercially available devices use solder that softens and fails at about 180xc2x0 C. The use of pure lead (Pb) solder in accordance with the invention is commercially useful for several reasons. First, the UL 1434 specification tests for PCL devices requires that the devices be heated above the melting point of the polymer and the melting point of the PFA polymer used in the PCL devices in accordance with the invention is about 300xc2x0 C. If a PCL device with attachment wires is to pass the UL specification test the joint between the device electrodes and the attachment wires must be able to withstand ambient temperatures above 300xc2x0 C. The pure lead (Pb) solder used on the PCL devices is cheap, easy to apply, consistent with current PCL device manufacturing technology, and works fine for a reasonable temperature range above 300xc2x0 C.
In accordance with the invention, the PCL devices do not need to have the polymer crosslinked to obtain stable electrical properties. In contrast, most commercially available PCL devices have to undergo a manufacturing step where the polymer is crosslinked. Radiation or chemical processes may be used to crosslink the conventional polymers. The crosslinking is claimed to give more stable and reliable devices. The PCL devices in accordance with the invention appear to be stable and reliable in all experiments in part because the polymer used in the devices is a fluoropolymer, which is composed mainly of carbon and fluorine atoms, compared to the non-fluoropolymers in conventional PCL devices, which are composed mainly of carbon and hydrogen atoms. Fluoropolymers are known to have significantly better chemical, oxidizer, and solvent resistance, better weatherability, and lower coefficient of water absorption than non-fluoropolymers. The fluoropolymer also has higher melt viscosity than the polymers used in other PCL devices, which may make it more stable during the melting process which occurs every time a device trips. The molecular weight of the polymer molecules may also be higher, making the polymer more stable.
In accordance with the invention, the PCL devices have higher trip and hold currents for a given device geometry and composite resistivity, than any other current limiting devices. This is a direct consequence of their having a higher melting point polymer since it takes more energy to get them to trip. This is commercially important because it allows the use of smaller devices in place of larger devices with no sacrifice in the magnitude of the trip and hold currents. This is potentially very useful in reducing the size of electronics. Raychem has been introducing progressively smaller sized current limiting devices over the past few years in particular for surface mount devices. When the current limiting devices are used in a surface mount configuration, their size takes up valuable real estate (the xe2x80x9cfootprintxe2x80x9d) on a circuit board. Raychem has so far attacked this problem by decreasing the resistivity of their standard polyethylene/carbon-black composite wherein decreasing the composite resistivity has the effect of increasing the trip and hold currents. However, there is little room for further decreases in composite resistivity using carbon black and polymers. The composite resistivity of the prototype PCL device in accordance with the invention is about 4 Ohm-cm while the Raychem devices have resistivities as low as 0.4 Ohm-cm. In principle, the resistivity of the PCL devices in accordance with the invention could be decreased to similar levels, which could give smaller devices with the same trip and hold currents as larger Raychem devices.